1. Field of the Invention.
The present invention relates to a heat transferable label and improved release composition therefor.
2. Description of the Prior Art.
Prior art heat transferable labels for imprinting designs onto an article typically involve decorative laminates consisting of a paper base sheet or web coated with a wax or polymeric release layer over which a design is imprinted in ink. The ink design is typically overcoated with a heat activatable adhesive layer.
Heat tranferable laminates having a wax based release layer, have the property that they soften readily at the desired heat transfer temperature, allowing the ink design layer and adhesive layer to transfer to the receiving article. The wax based release, during heat transfer, splits (i.e., bifurcates) from the carrier and a uniform portion of the release transfers along with the ink design layer to provide a protective coating over the ink layer upon transfer to the receiving article being decorated. Wax based release, however, after transfer to the receiving article, can be abraded when exposed to high degree of scuffing. They are also subject to attack by solvents to which the laminate may be exposed after transfer to the receiving article.
U.S. Pat. No. 3,616,015 is illustrative of the prior art. In U.S. Pat. No. 3,616,015 a lebal carrying web such as a paper sheet includes a heat transferable label composed of a wax release layer affixed to a surface of the paper sheet and an ink design layer superimposed onto the wax release layer. In the heat transfer labeling process for imprinting designs onto articles, the label carrying web is subjected to heat and the laminate is pressed onto an article with the ink design layer making direct contact with the article. As the web or paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the wax layer. After transfer of the design to the article, the paper sheet is immediately removed, leaving the design firmly affixed to the surface with the wax layer exposed to the environment. The reference teaches that the wax release layer must not only permit release of the transferable label from the web upon application of heat to the web, but also must form a clear protective layer over the transferred ink design.
This commonly assigned patent discloses a wax release coating containing a modified montan wax which has been oxidized, esterfied, and partially saponified. In order to attain improved clarity of the transferred ink design the transferred wax coating over the ink design is subjected to additional heat processing after the label has been transferred onto an article. The additional processing involves postflaming, wherein the transferred wax coating is subjected to jets of high temperature gas either as direct gas flame or as hot air jets at temperatures of about 300.degree. F. to 400.degree. F. for a period of time sufficient to remelt the wax coating without substantially heating the bottle. Upon cooling of the remelted wax coating through use of ambient or forced cooled air, the cooled wax layer solidifies to form a clear, smooth protective coating over the transferred ink design. Although the heat transferable label and process disclosed in U.S. Pat. No. 3,616,015 represents an improvement over prior heat transferable labels, they are best suited to decoration of plastic articles which are not transparent. Although the heat transferable label disclosed in this reference may be utilized for decorating a wide variety of different plastics, there can be a degree of hazing or halo noticeable over the transferred label when the transfer is made onto clear plastic materials, despite use of postflaming.
U.S. Pat. No. 3,922,435 discloses a dry nonwax release heat transfer laminate to be transferred to plastic bottles. The laminate is composed of a backing sheet coated with a dry release layer which, in turn, is overcoated with an ink design layer and adhesive layer. The release layer is preferably composed of a cross-linked thermoset polymeric resin and does not contain wax components. Preferably the thermoset resin in the release layer is a cross-linked resin selected from acrylic resins, polyamide resins, polyester resins, vinyl resins, epoxy resins, epoxy-acrylate resins, allyl resins, aldehyde resins, such as phenol-formaldehyde resins and the aminoaldehyde resins, e.g., urea formaldehyde or melamine aldehyde. The release layer disclosed in this reference may also be the theremoplastic polypropylene which has the requisite softening point and adhesive characteristics. As the exposed surface of the laminate contacts a bottle or article to be decorated and heat typically between about 300.degree. F. to 450.degree. F. is applied to the backing, the dry release layer has the property that allows a clean peel of the ink design layer from it so that the ink design layer transfers to the article. The dry release as in U.S. Pat. No. 3,922,435 does not contain a wax component as in U.S. Pat. No. 3,616,015 and therefore functions differently than a wax based release. A wax based release as in U.S. Pat. No. 3,616,015 softens and splits on transfer. Thus, a portion of the wax release is left behind on the backing and the remainder transfers to the receiving article along with the ink design to form a protective coating thereon. The dry release as disclosed in U.S. Pat. No. 3,922,435 in contrast does not melt, i.e., remains "dry" and separates cleanly from the ink design layer on application of heat. The dry release does not transfer to the article being decorated, rather all of the dry release is left behind on the paper backing. The thermoset resins disclosed in this reference are applied to the backing sheet from a solution containing curing catalyst. After the solution containing the thermoset resin is coated onto the backing it is cured at curing temperatures typically between 250.degree. F. to 350.degree. F. to achieve cross linking of the polymer. The remaining layers including the ink design layer are subsequently coated over the dry release to form the laminate.
U.S. Pat. No. 2,990,311 discloses a heat transferable decal having a release transfer layer composed of a mixture of a crystalline wax and a synthetic thermoplastic film-forming resin, principally an organic linear thermoplastic film-forming resin which is substantially water insoluble. The degree of compatibility of the resin and wax is controlled through selection and ratio of the components to give heat transfers of either the hot peel or cold peel type. In the hot peel transfer, the decal will adhere and release from the backing only immediately after application while the decal is still hot. In the cold peel transfer the transferred decal will adhere to the receiving surface when hot but will only release and transfer by peeling away the backing after the transfer has cooled. In either type of transfer, this reference teaches that resins and waxes (the latter being used for the release layer) should be mutually incompatible or insoluble at temperatures below the melting temperature of the wax such that the molten wax, upon cooling, will acutally crystallize separately and distinctly from the resin. The resins are linear thermoplastic film-forming resins defined as essentially solvent-soluble, softened by heat and to involve only a minor amount or no cross-linkage.
U.S. Pat. No. 2,862,832 discloses a heat transferable decal having a release layer composed of an oxidized wax. The disclosure is directed principally to defining the type of wax found to provide suitable release of the decal from the carrier web upon application of heat. The wax disclosed in this reference is an oxidized wax obtained as the reaction product of the oxidiation of hard, high melting, aliphatic, hydrocarbon waxes. The oxidized waxes are defined as the oxidation products of both natural and synthetic hydrocarbon waxes such as petroleum waxes, low molecular weight polyethylene and waxes obtained from the Fischer-Tropsch synthesis. Suitable wax may include oxidized microcrystalline wax or the esterification product of an oxidized hydrocarbon wax. The oxidized waxes are disclosed as those having melting points between about 50.degree. C. and 110.degree. C., saponification values between about 25 and 100, acid values between about 5 and 40, and penetrometer hardness (ASTM D5-52) below about 51 as measured with 100 grams for 5 seconds at 25.degree. C.
In U.S. Pat. No. 3,616,176 the laminate is composed of a base sheet, with a polyamide layer covering the base sheet and a decorative ink layer covering the polyamide layer. Sufficient heat is applied to the laminate to heat the polyamide layer at or above a softening point, and the laminate is then pressed onto the surface of an article with the decorative ink layer coming into direct contact. Upon withdrawl of the heat source, the polyamide layer cools to a temperature below the softening point and the base sheet is removed. The decorative layer becomes fused or heat sealed to the article. The polyamide layer in this disclosure functions as a release coating which allows transfer of the decorative layer onto an article and upon cooling serves as a protective coating layer over the transferred decorative layer. The use of a polyamide release coating has the principal disadvantage in that there is a significant tendency for the polyamide to form a noticeable halo about the transferred decorative layer. Also the polyamide layer, even when subjected to additional processing such as postflaming, does not form sufficiently clear coating that would esthetically permit heat transfer labelling onto clear articles or bottles.
U.S. Pat. No. 4,557,964, 4,581,266, and 4,536,434 are additional prior art references which discloses a heat transferable laminate utilizing a wax based release layer. Specific classes of tackifying agents are added to the wax release to improve the clarity and function of the release coat after transfer of the laminate to an article whereupon the release functions as a protective layer over the ink design. The release compositions disclosed in these references are of the wax based type and therefore are not applicable to the formulations disclosed in the present patent application.
Accordingly, it is an object of the present invention to provide a nonwax based release for heat transfer laminates wherein the release exhibits improved characteristics of abrasion and scuff resistance, and resistance to attack by solvents to which the laminate may come into contact after transfer to the receiving article.
It is an important object of the present invention that the nonwax based release function as a wax release on application of heat to the laminate. Thus, it is important that the nonwax release split on application of heat thus causing a uniform portion of the release to transfer to the receiving article along with the ink design layer and completely cover the ink layer on transfer.
It is a further important object that the nonwax release be curable by ultraviolet light after transfer to the article.